This invention is directed to sample liquid aspiration and dispensing systems and more particularly to a novel conductive aspiration and dispensing probe for repetitive sample taking which has minimal carryover from previously aspirated and dispensed samples.
Samples of a liquid specimen such as serum that are to be tested or analyzed are usually drawn from a single source such as a sample tube. In immuno-assay testing of serum it is common practice to conduct four to six different tests and each test generally requires the drawing of a separate sample. The repetitive drawing of serum samples and the processing of such samples in the reagents used for testing can be extremely time-consuming if such tests are performed by a technician in a laboratory. Since the demand for immuno-acid serum testing has steadily increased it has become necessary to automate almost all aspects of such testing, including the repetitive drawing of samples.
An automated sampling system such as shown in U.S. Pat. No. 5,133,218 operates with a sampling probe that is used for repetitive sample taking. Carryover of sample and reagent from one aspiration to another is substantially reduced to acceptable limits by automatically rinsing the probe after each sample is aspirated and dispensed. Thus, the probe is relatively free of carryover during repetitive aspirations and dispensations of sample, even when such samples are taken from different sources such as different sampling tubes.
The probe of U.S. Pat. No. 5,133,218 is non-conductive and has at least two sections of different diameter. The junction between the sections of different diameter is often the site of turbulent eddies that form in the rinse liquid as it flows through the probe. Turbulent eddies interfere with the flushing action of the rinse liquid and thus reduce the effectiveness of a rinse cycle.
Other known aspirating and dispensing probes such as shown in U.S. Pat. Nos. 4,210,156; 4,707,337; 5,073,347; and 5,232,669 are generally formed by a molding process. However, a molded surface usually has imperfections or irregularities of the type that can cause the formation of turbulent eddies at almost any point along the inner surface of the probe when a rinse liquid flows through the probe.
It is also known to coat the inner surface of a probe, especially if it is formed of metal, with a plastic material to smooth out any imperfections in the molded surface. However, surface flaws in a plastic coating are common and will give rise to turbulent eddy conditions in rinse liquid that flows along the coated surface during a rinse cycle.
The drawing down of a plastic tube using dies is a preferred method for making a probe having two or more sections of different diameter since the draw-down process can be controlled to preserve the smoothness of the inner surface of the tube and such surface smoothness is superior to that of a molded or coated surface.
Surface smoothness in a die-drawn tube is usually fine enough to avert the formation of turbulent eddies in a current of rinse liquid. However, the junction between diametrical reductions in a die-drawn tube can be the site of turbulent eddy formation when rinse liquid flows past the junction.
It is thus desirable to provide a sample liquid aspirating and dispensing probe which has optimal inner surface smoothness and a streamlined profile to avert the formation of turbulent eddies when rinse liquid flows through the probe. It is also desirable to render such probe conductive to permit capacitive liquid level sensing.